Neurokinins produce selective venoconstriction via NK-3 receptors in the rat mesenteric vascular bed

The vasoactive properties of the neurokinins (substance P (SP), neurokinin A (NKA), neurokinin B (NKB)) and some selective analogues were assessed in the arterial and venous mesenteric beds of the rat. Although both sides of the mesenteric vasculature displayed endothclium-dependent relaxation in response to acctyloholine (ACh) or bradykinin (BK) (1 and 10 nmol), SP and the selective NK-1 analogue, [Sar 9,Met(O 2) 11]SP were inactive. Of the three selective neurokinin agonists used, (Sar 9,Met(O 2) 11]SP (NK-1), [β-Ala 8]NKA-(4–10) (NK-2) and [McPhe 7]NKB (NK-3), only the latter induced a dose-dependent pressor effect in the venous mesenteric vasculature. Injections of SP and the selective NK-1 and NK-2 analogues at high doses (10 nmol), did not change the perfusion pressure in the mesenteric bed even when the mesenteric vasculature was treated with methylene blue (50 μM) to inhibit the effects of endothelium-derived relaxing factor (EDRF) or with N G-nitro-L-arginine (L-NNA) (20 μM) to inhibit the formation of EDRF or with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate] (CHAPS 20 mM, 30 s) to remove the endothelial layer. In contrast, the vasoconstrictor effects of noradrenaline (NA), angiotensin II (AT 11), NKB and [MePhe 7]NKB on the venous side of the circulation were enhanced following treatment with L-NNA, methylene blue or CHAPS. The present results suggest that neurokinins act on the rat mesenteric bed by increasing the perfusion pressure of the venous vasculature via activation of NK-3 receptors. Neurokinins are inactive on the arterial mesenteric vasculature. The endothelial layer of the venous vasculature may interfere by acting as a physical and physiological barrier (basal release of EDRF) to the pressor response induced by NA, at 11, NKB and [MePhe 7]NKB.